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Stanford scientists have found evidence that sections of the fault responsible for the 9.0 magnitude Tohoku earthquake that devastated northern Japan in 2011 were relieving seismic stress at a gradually accelerating rate for years before the quake.

This “decoupling” process, in which the edges of two tectonic plates that are frictionally locked together slowly became unstuck, transferred stress to adjacent sections that were still locked. As a result, the quake, which was the most powerful ever recorded to hit Japan, may have occurred earlier than it might have otherwise, said Andreas Mavrommatis, a graduate student in Stanford’s School of Earth Sciences.

Mavrommatis and his advisor, Paul Segall, a professor of geophysics at Stanford, reached their conclusions after analyzing 15 years’ worth of GPS measurements from the Japanese island of Honshu. Their results were published earlier this year in the journal Geophysical Research Letters.

“We looked at northeastern Japan, which has one of the densest and longest running high-precision GPS networks in the world,” Mavrommatis said.

Segall said, “The measurements indicated the plate boundary was gradually becoming less locked over time. That was surprising.”

The scientists will present their work, “Decadal-Scale Decoupling of the Japan Trench Prior to the 2011 Tohoku-Oki Earthquake from Geodetic and Repeating-Earthquake Observations,” Dec. 17 at the American Geophysical Union’s Fall Meeting in San Francisco. The talk will take place at 5 p.m. PT at the Moscone Convention Center in Moscone South, Room 306.

The pair’s hypothesis is further supported by a recent analysis they conducted of so-called repeating earthquakes offshore of northern Honshu. The small quakes, which were typically magnitude 3 or 4, occurred along the entire length of the fault line, but each one occurred at the same spot every few years. Furthermore, many of them were repeating not at a constant but an accelerating rate, the scientists found. This acceleration would be expected if the fault were becoming less locked over time, Mavrommatis said, because the decoupling process would have relieved pent-up stress along some sections of the fault but increased stress on adjacent sections.

“According to our model, the decoupling process would have had the effect of adding stress to the section of the fault that nucleated the Tohoku quake,” Segall said. “We suspect this could have accelerated the occurrence of the earthquake.”

The scientists caution that their results cannot be used to predict the occurrence of the next major earthquake in Japan, but it could shed light on the physical processes that operate on faults that generate the world’s largest quakes.

Stanford scientists have found evidence that sections of the fault responsible for the 9.0 magnitude Tohoku earthquake that devastated northern Japan in 2011 were relieving seismic stress at a gradually accelerating rate for years before the quake.

This “decoupling” process, in which the edges of two tectonic plates that are frictionally locked together slowly became unstuck, transferred stress to adjacent sections that were still locked. As a result, the quake, which was the most powerful ever recorded to hit Japan, may have occurred earlier than it might have otherwise, said Andreas Mavrommatis, a graduate student in Stanford’s School of Earth Sciences.

Mavrommatis and his advisor, Paul Segall, a professor of geophysics at Stanford, reached their conclusions after analyzing 15 years’ worth of GPS measurements from the Japanese island of Honshu. Their results were published earlier this year in the journal Geophysical Research Letters.

“We looked at northeastern Japan, which has one of the densest and longest running high-precision GPS networks in the world,” Mavrommatis said.

Segall said, “The measurements indicated the plate boundary was gradually becoming less locked over time. That was surprising.”

The scientists will present their work, “Decadal-Scale Decoupling of the Japan Trench Prior to the 2011 Tohoku-Oki Earthquake from Geodetic and Repeating-Earthquake Observations,” Dec. 17 at the American Geophysical Union’s Fall Meeting in San Francisco. The talk will take place at 5 p.m. PT at the Moscone Convention Center in Moscone South, Room 306.

The pair’s hypothesis is further supported by a recent analysis they conducted of so-called repeating earthquakes offshore of northern Honshu. The small quakes, which were typically magnitude 3 or 4, occurred along the entire length of the fault line, but each one occurred at the same spot every few years. Furthermore, many of them were repeating not at a constant but an accelerating rate, the scientists found. This acceleration would be expected if the fault were becoming less locked over time, Mavrommatis said, because the decoupling process would have relieved pent-up stress along some sections of the fault but increased stress on adjacent sections.

“According to our model, the decoupling process would have had the effect of adding stress to the section of the fault that nucleated the Tohoku quake,” Segall said. “We suspect this could have accelerated the occurrence of the earthquake.”

The scientists caution that their results cannot be used to predict the occurrence of the next major earthquake in Japan, but it could shed light on the physical processes that operate on faults that generate the world’s largest quakes.

Traditional thought holds that hot updrafts from the Earth’s core cause volcanoes, but researchers say eruptions may stem from the asthenosphere, a layer closer to the surface. – Virginia Tech

A long-held assumption about the Earth is discussed in today’s edition of Science, as Don L. Anderson, an emeritus professor with the Seismological Laboratory of the California Institute of Technology, and Scott King, a professor of geophysics in the College of Science at Virginia Tech, look at how a layer beneath the Earth’s crust may be responsible for volcanic eruptions.

The discovery challenges conventional thought that volcanoes are caused when plates that make up the planet’s crust shift and release heat.

Instead of coming from deep within the interior of the planet, the responsibility is closer to the surface, about 80 kilometers to 200 kilometers deep — a layer above the Earth’s mantle, known as the as the asthenosphere.

“For nearly 40 years there has been a debate over a theory that volcanic island chains, such as Hawaii, have been formed by the interaction between plates at the surface and plumes of hot material that rise from the core-mantle boundary nearly 1,800 miles below the Earth’s surface,” King said. “Our paper shows that a hot layer beneath the plates may explain the origin of mid-plate volcanoes without resorting to deep conduits from halfway to the center of the Earth.”

Traditionally, the asthenosphere has been viewed as a passive structure that separates the moving tectonic plates from the mantle.

As tectonic plates move several inches every year, the boundaries between the plates spawn most of the planet’s volcanoes and earthquakes.

“As the Earth cools, the tectonic plates sink and displace warmer material deep within the interior of the Earth,” explained King. “This material rises as two broad, passive updrafts that seismologists have long recognized in their imaging of the interior of the Earth.”

The work of Anderson and King, however, shows that the hot, weak region beneath the plates acts as a lubricating layer, preventing the plates from dragging the material below along with them as they move.

The researchers show this lubricating layer is also the hottest part of the mantle, so there is no need for heat to be carried up to explain mid-plate volcanoes.

“We’re taking the position that plate tectonics and mid-plate volcanoes are the natural results of processes in the plates and the layer beneath them,” King said.

Traditional thought holds that hot updrafts from the Earth’s core cause volcanoes, but researchers say eruptions may stem from the asthenosphere, a layer closer to the surface. – Virginia Tech

A long-held assumption about the Earth is discussed in today’s edition of Science, as Don L. Anderson, an emeritus professor with the Seismological Laboratory of the California Institute of Technology, and Scott King, a professor of geophysics in the College of Science at Virginia Tech, look at how a layer beneath the Earth’s crust may be responsible for volcanic eruptions.

The discovery challenges conventional thought that volcanoes are caused when plates that make up the planet’s crust shift and release heat.

Instead of coming from deep within the interior of the planet, the responsibility is closer to the surface, about 80 kilometers to 200 kilometers deep — a layer above the Earth’s mantle, known as the as the asthenosphere.

“For nearly 40 years there has been a debate over a theory that volcanic island chains, such as Hawaii, have been formed by the interaction between plates at the surface and plumes of hot material that rise from the core-mantle boundary nearly 1,800 miles below the Earth’s surface,” King said. “Our paper shows that a hot layer beneath the plates may explain the origin of mid-plate volcanoes without resorting to deep conduits from halfway to the center of the Earth.”

Traditionally, the asthenosphere has been viewed as a passive structure that separates the moving tectonic plates from the mantle.

As tectonic plates move several inches every year, the boundaries between the plates spawn most of the planet’s volcanoes and earthquakes.

“As the Earth cools, the tectonic plates sink and displace warmer material deep within the interior of the Earth,” explained King. “This material rises as two broad, passive updrafts that seismologists have long recognized in their imaging of the interior of the Earth.”

The work of Anderson and King, however, shows that the hot, weak region beneath the plates acts as a lubricating layer, preventing the plates from dragging the material below along with them as they move.

The researchers show this lubricating layer is also the hottest part of the mantle, so there is no need for heat to be carried up to explain mid-plate volcanoes.

“We’re taking the position that plate tectonics and mid-plate volcanoes are the natural results of processes in the plates and the layer beneath them,” King said.

If Brad Singer knew for sure what was happening three miles under an odd-shaped lake in the Andes, he might be less eager to spend a good part of his career investigating a volcanic field that has erupted 36 times during the last 25,000 years. As he leads a large scientific team exploring a region in the Andes called Laguna del Maule, Singer hopes the area remains quiet.

But the primary reason to expend so much effort on this area boils down to one fact: The rate of uplift is among the highest ever observed by satellite measurement for a volcano that is not actively erupting.

That uplift is almost definitely due to a large intrusion of magma — molten rock — beneath the volcanic complex. For seven years, an area larger than the city of Madison has been rising by 10 inches per year.

That rapid rise provides a major scientific opportunity: to explore a mega-volcano before it erupts. That effort, and the hazard posed by the restless magma reservoir beneath Laguna del Maule, are described in a major research article in the December issue of the Geological Society of America’s GSA Today.

“We’ve always been looking at these mega-eruptions in the rear-view mirror,” says Singer. “We look at the lava, dust and ash, and try to understand what happened before the eruption. Since these huge eruptions are rare, that’s usually our only option. But we look at the steady uplift at Laguna del Maule, which has a history of regular eruptions, combined with changes in gravity, electrical conductivity and swarms of earthquakes, and we suspect that conditions necessary to trigger another eruption are gathering force.”

Laguna del Maule looks nothing like a classic, cone-shaped volcano, since the high-intensity erosion caused by heavy rain and snow has carried most of the evidence to the nearby Pacific Ocean. But the overpowering reason for the absence of “typical volcano cones” is the nature of the molten rock underground. It’s called rhyolite, and it’s the most explosive type of magma on the planet.

The eruption of a rhyolite volcano is too quick and violent to build up a cone. Instead, this viscous, water-rich magma often explodes into vast quantities of ash that can form deposits hundreds of yards deep, followed by a slower flow of glassy magma that can be tens of yards tall and measure more than a mile in length.

The next eruption could be in the size range of Mount St. Helens — or it could be vastly bigger, Singer says. “We know that over the past million years or so, several eruptions at Laguna del Maule or nearby volcanoes have been more than 100 times larger than Mount St. Helens,” he says. “Those are rare, but they are possible.” Such a mega-eruption could change the weather, disrupt the ecosystem and damage the economy.

Trying to anticipate what Laguna del Maule holds in store, Singer is heading a new $3 million, five-year effort sponsored by the National Science Foundation to document its behavior before an eruption. With colleagues from Chile, Argentina, Canada, Singapore, and Cornell and Georgia Tech universities, he is masterminding an effort to build a scientific model of the underground forces that could lead to eruption. “This model should capture how this system has evolved in the crust at all scales, from the microscopic to basinwide, over the last 100,000 years,” Singer says. “It’s like a movie from the past to the present and into the future.”

Over the next five years, Singer says he and 30 colleagues will “throw everything, including the kitchen sink, at the problem — geology, geochemistry, geochronology and geophysics — to help measure, and then model, what’s going on.”

One key source of information on volcanoes is seismic waves. Ground shaking triggered by the movement of magma can signal an impending eruption. Team member Clifford Thurber, a seismologist and professor of geoscience at UW-Madison, wants to use distant earthquakes to locate the underground magma body.

As many as 50 seismometers will eventually be emplaced above and around the magma at Laguna del Maule, in the effort to create a 3-D image of Earth’s crust in the area.

By tracking multiple earthquakes over several years, Thurber and his colleagues want to pinpoint the size and location of the magma body — roughly estimated as an oval measuring five kilometers (3.1 miles) by 10 kilometers (6.2 miles).

Each seismometer will record the travel time of earthquake waves originating within a few thousand kilometers, Thurber explains. Since soft rock transmits sound less efficiently than hard rock, “we expect that waves that pass through the presumed magma body will be delayed,” Thurber says. “It’s very simple. It’s like a CT scan, except instead of density we are looking at seismic wave velocity.”

As Singer, who has been visiting Laguna del Maule since 1998, notes, “The rate of uplift — among the highest ever observed — has been sustained for seven years, and we have discovered a large, fluid-rich zone in the crust under the lake using electrical resistivity methods. Thus, there are not many possible explanations other than a big, active body of magma at a shallow depth.”

The expanding body of magma could freeze in place — or blow its top, he says. “One thing we know for sure is that the surface cannot continue rising indefinitely.”

If Brad Singer knew for sure what was happening three miles under an odd-shaped lake in the Andes, he might be less eager to spend a good part of his career investigating a volcanic field that has erupted 36 times during the last 25,000 years. As he leads a large scientific team exploring a region in the Andes called Laguna del Maule, Singer hopes the area remains quiet.

But the primary reason to expend so much effort on this area boils down to one fact: The rate of uplift is among the highest ever observed by satellite measurement for a volcano that is not actively erupting.

That uplift is almost definitely due to a large intrusion of magma — molten rock — beneath the volcanic complex. For seven years, an area larger than the city of Madison has been rising by 10 inches per year.

That rapid rise provides a major scientific opportunity: to explore a mega-volcano before it erupts. That effort, and the hazard posed by the restless magma reservoir beneath Laguna del Maule, are described in a major research article in the December issue of the Geological Society of America’s GSA Today.

“We’ve always been looking at these mega-eruptions in the rear-view mirror,” says Singer. “We look at the lava, dust and ash, and try to understand what happened before the eruption. Since these huge eruptions are rare, that’s usually our only option. But we look at the steady uplift at Laguna del Maule, which has a history of regular eruptions, combined with changes in gravity, electrical conductivity and swarms of earthquakes, and we suspect that conditions necessary to trigger another eruption are gathering force.”

Laguna del Maule looks nothing like a classic, cone-shaped volcano, since the high-intensity erosion caused by heavy rain and snow has carried most of the evidence to the nearby Pacific Ocean. But the overpowering reason for the absence of “typical volcano cones” is the nature of the molten rock underground. It’s called rhyolite, and it’s the most explosive type of magma on the planet.

The eruption of a rhyolite volcano is too quick and violent to build up a cone. Instead, this viscous, water-rich magma often explodes into vast quantities of ash that can form deposits hundreds of yards deep, followed by a slower flow of glassy magma that can be tens of yards tall and measure more than a mile in length.

The next eruption could be in the size range of Mount St. Helens — or it could be vastly bigger, Singer says. “We know that over the past million years or so, several eruptions at Laguna del Maule or nearby volcanoes have been more than 100 times larger than Mount St. Helens,” he says. “Those are rare, but they are possible.” Such a mega-eruption could change the weather, disrupt the ecosystem and damage the economy.

Trying to anticipate what Laguna del Maule holds in store, Singer is heading a new $3 million, five-year effort sponsored by the National Science Foundation to document its behavior before an eruption. With colleagues from Chile, Argentina, Canada, Singapore, and Cornell and Georgia Tech universities, he is masterminding an effort to build a scientific model of the underground forces that could lead to eruption. “This model should capture how this system has evolved in the crust at all scales, from the microscopic to basinwide, over the last 100,000 years,” Singer says. “It’s like a movie from the past to the present and into the future.”

Over the next five years, Singer says he and 30 colleagues will “throw everything, including the kitchen sink, at the problem — geology, geochemistry, geochronology and geophysics — to help measure, and then model, what’s going on.”

One key source of information on volcanoes is seismic waves. Ground shaking triggered by the movement of magma can signal an impending eruption. Team member Clifford Thurber, a seismologist and professor of geoscience at UW-Madison, wants to use distant earthquakes to locate the underground magma body.

As many as 50 seismometers will eventually be emplaced above and around the magma at Laguna del Maule, in the effort to create a 3-D image of Earth’s crust in the area.

By tracking multiple earthquakes over several years, Thurber and his colleagues want to pinpoint the size and location of the magma body — roughly estimated as an oval measuring five kilometers (3.1 miles) by 10 kilometers (6.2 miles).

Each seismometer will record the travel time of earthquake waves originating within a few thousand kilometers, Thurber explains. Since soft rock transmits sound less efficiently than hard rock, “we expect that waves that pass through the presumed magma body will be delayed,” Thurber says. “It’s very simple. It’s like a CT scan, except instead of density we are looking at seismic wave velocity.”

As Singer, who has been visiting Laguna del Maule since 1998, notes, “The rate of uplift — among the highest ever observed — has been sustained for seven years, and we have discovered a large, fluid-rich zone in the crust under the lake using electrical resistivity methods. Thus, there are not many possible explanations other than a big, active body of magma at a shallow depth.”

The expanding body of magma could freeze in place — or blow its top, he says. “One thing we know for sure is that the surface cannot continue rising indefinitely.”

Images of the ice-covered Gamburtsev Mountains revealed water-filled valleys, as seen by the cluster of vertical lines in this image. – Tim Creyts

Time ravages mountains, as it does people. Sharp features soften, and bodies grow shorter and rounder. But under the right conditions, some mountains refuse to age. In a new study, scientists explain why the ice-covered Gamburtsev Mountains in the middle of Antarctica looks as young as they do.

The Gamburtsevs were discovered in the 1950s, but remained unexplored until scientists flew ice-penetrating instruments over the mountains 60 years later. As this ancient hidden landscape came into focus, scientists were stunned to see the saw-toothed and towering crags of much younger mountains. Though the Gamburtsevs are contemporaries of the largely worn-down Appalachians, they looked more like the Rockies, which are nearly 200 million years younger.

More surprising still, the scientists discovered a vast network of lakes and rivers at the mountains’ base. Though water usually speeds erosion, here it seems to have kept erosion at bay. The reason, researchers now say, has to do with the thick ice that has entombed the Gamburtsevs since Antarctica went into a deep freeze 35 million years ago.

“The ice sheet acts like an anti-aging cream,” said the study’s lead author, Timothy Creyts, a geophysicist at Columbia University’s Lamont-Doherty Earth Observatory. “It triggers a series of thermodynamic processes that have almost perfectly preserved the Gamburtsevs since ice began spreading across the continent.”

The study, which appears in the latest issue of the journal Geophysical Research Letters, explains how the blanket of ice covering the Gamburtsevs has preserved its rugged ridgelines.

Snow falling at the surface of the ice sheet draws colder temperatures down, closer to protruding peaks in a process called divergent cooling. At the same time, heat radiating from bedrock beneath the ice sheet melts ice in the deep valleys to form rivers and lakes. As rivers course along the base of the ice sheet, high pressures from the overlying ice sheet push water up valleys in reverse. This uphill flow refreezes as it meets colder temperature from above. Thus, ridgelines are cryogenically preserved.

The oldest rocks in the Gamburtsevs formed more than a billion years ago, in the collision of several continents. Though these prototype mountains eroded away, a lingering crustal root became reactivated when the supercontinent Gondwana ripped apart, starting about 200 million years ago. Tectonic forces pushed the land up again to form the modern Gamburtsevs, which range across an area the size of the Alps. Erosion again chewed away at the mountains until earth entered a cooling phase 35 million years ago. Expanding outward from the Gamburtsevs, a growing layer of ice joined several other nucleation points to cover the entire continent in ice.

The researchers say that the mechanism that stalled aging of the Gamburtsevs at higher elevations may explain why some ridgelines in the Torngat Mountains on Canada’s Labrador Peninsula and the Scandinavian Mountains running through Norway, Sweden and Finland appear strikingly untouched. Massive ice sheets covered both landscapes during the last ice age, which peaked about 20,000 years ago, but many high-altitude features bear little trace of this event.

“The authors identify a mechanism whereby larger parts of mountains ranges in glaciated regions–not just Antarctica–could be spared from erosion,” said Stewart Jamieson, a glaciologist at Durham University who was not involved in the study. “This is important because these uplands are nucleation centers for ice sheets. If they were to gradually erode during glacial cycles, they would become less effective as nucleation points during later ice ages.”

Ice sheet behavior, then, may influence climate change in ways that scientists and computer models have yet to appreciate. As study coauthor Fausto Ferraccioli, head of the British Antarctic Survey’s airborne geophysics group, put it: “If these mountains in interior East Antarctica had been more significantly eroded then the ice sheet itself
may have had a different history.”

Other Authors

Hugh Carr and Tom Jordan of the British Antarctic Survey; Robin Bell, Michael Wolovick and Nicholas Frearson of Lamont-Doherty; Kathryn Rose of University of Bristol; Detlef Damaske of Germany’s Federal Institute for Geosciences and Natural Resources; David Braaten of Kansas University; and Carol Finn of the U.S. Geological Survey.

Copies of the paper, “Freezing of ridges and water networks preserves the Gamburtsev Subglacial Mountains for millions of years,” are available from the authors.

Images of the ice-covered Gamburtsev Mountains revealed water-filled valleys, as seen by the cluster of vertical lines in this image. – Tim Creyts

Time ravages mountains, as it does people. Sharp features soften, and bodies grow shorter and rounder. But under the right conditions, some mountains refuse to age. In a new study, scientists explain why the ice-covered Gamburtsev Mountains in the middle of Antarctica looks as young as they do.

The Gamburtsevs were discovered in the 1950s, but remained unexplored until scientists flew ice-penetrating instruments over the mountains 60 years later. As this ancient hidden landscape came into focus, scientists were stunned to see the saw-toothed and towering crags of much younger mountains. Though the Gamburtsevs are contemporaries of the largely worn-down Appalachians, they looked more like the Rockies, which are nearly 200 million years younger.

More surprising still, the scientists discovered a vast network of lakes and rivers at the mountains’ base. Though water usually speeds erosion, here it seems to have kept erosion at bay. The reason, researchers now say, has to do with the thick ice that has entombed the Gamburtsevs since Antarctica went into a deep freeze 35 million years ago.

“The ice sheet acts like an anti-aging cream,” said the study’s lead author, Timothy Creyts, a geophysicist at Columbia University’s Lamont-Doherty Earth Observatory. “It triggers a series of thermodynamic processes that have almost perfectly preserved the Gamburtsevs since ice began spreading across the continent.”

The study, which appears in the latest issue of the journal Geophysical Research Letters, explains how the blanket of ice covering the Gamburtsevs has preserved its rugged ridgelines.

Snow falling at the surface of the ice sheet draws colder temperatures down, closer to protruding peaks in a process called divergent cooling. At the same time, heat radiating from bedrock beneath the ice sheet melts ice in the deep valleys to form rivers and lakes. As rivers course along the base of the ice sheet, high pressures from the overlying ice sheet push water up valleys in reverse. This uphill flow refreezes as it meets colder temperature from above. Thus, ridgelines are cryogenically preserved.

The oldest rocks in the Gamburtsevs formed more than a billion years ago, in the collision of several continents. Though these prototype mountains eroded away, a lingering crustal root became reactivated when the supercontinent Gondwana ripped apart, starting about 200 million years ago. Tectonic forces pushed the land up again to form the modern Gamburtsevs, which range across an area the size of the Alps. Erosion again chewed away at the mountains until earth entered a cooling phase 35 million years ago. Expanding outward from the Gamburtsevs, a growing layer of ice joined several other nucleation points to cover the entire continent in ice.

The researchers say that the mechanism that stalled aging of the Gamburtsevs at higher elevations may explain why some ridgelines in the Torngat Mountains on Canada’s Labrador Peninsula and the Scandinavian Mountains running through Norway, Sweden and Finland appear strikingly untouched. Massive ice sheets covered both landscapes during the last ice age, which peaked about 20,000 years ago, but many high-altitude features bear little trace of this event.

“The authors identify a mechanism whereby larger parts of mountains ranges in glaciated regions–not just Antarctica–could be spared from erosion,” said Stewart Jamieson, a glaciologist at Durham University who was not involved in the study. “This is important because these uplands are nucleation centers for ice sheets. If they were to gradually erode during glacial cycles, they would become less effective as nucleation points during later ice ages.”

Ice sheet behavior, then, may influence climate change in ways that scientists and computer models have yet to appreciate. As study coauthor Fausto Ferraccioli, head of the British Antarctic Survey’s airborne geophysics group, put it: “If these mountains in interior East Antarctica had been more significantly eroded then the ice sheet itself
may have had a different history.”

Other Authors

Hugh Carr and Tom Jordan of the British Antarctic Survey; Robin Bell, Michael Wolovick and Nicholas Frearson of Lamont-Doherty; Kathryn Rose of University of Bristol; Detlef Damaske of Germany’s Federal Institute for Geosciences and Natural Resources; David Braaten of Kansas University; and Carol Finn of the U.S. Geological Survey.

Copies of the paper, “Freezing of ridges and water networks preserves the Gamburtsev Subglacial Mountains for millions of years,” are available from the authors.

A new analysis from The University of Texas at Austin’s Institute for Geophysics suggests a deep oceanic gateway, shown in blue, developed between the Pacific and Iapetus oceans immediately before the Cambrian sea level rise and explosion of life in the fossil record, isolating Laurentia from the supercontinent Gondwanaland. – Ian Dalziel

A new analysis of geologic history may help solve the riddle of the “Cambrian explosion,” the rapid diversification of animal life in the fossil record 530 million years ago that has puzzled scientists since the time of Charles Darwin.

A paper by Ian Dalziel of The University of Texas at Austin’s Jackson School of Geosciences, published in the November issue of Geology, a journal of the Geological Society of America, suggests a major tectonic event may have triggered the rise in sea level and other environmental changes that accompanied the apparent burst of life.

The Cambrian explosion is one of the most significant events in Earth’s 4.5-billion-year history. The surge of evolution led to the sudden appearance of almost all modern animal groups. Fossils from the Cambrian explosion document the rapid evolution of life on Earth, but its cause has been a mystery.

The sudden burst of new life is also called “Darwin’s dilemma” because it appears to contradict Charles Darwin’s hypothesis of gradual evolution by natural selection.

“At the boundary between the Precambrian and Cambrian periods, something big happened tectonically that triggered the spreading of shallow ocean water across the continents, which is clearly tied in time and space to the sudden explosion of multicellular, hard-shelled life on the planet,” said Dalziel, a research professor at the Institute for Geophysics and a professor in the Department of Geological Sciences.

Beyond the sea level rise itself, the ancient geologic and geographic changes probably led to a buildup of oxygen in the atmosphere and a change in ocean chemistry, allowing more complex life-forms to evolve, he said.

The paper is the first to integrate geological evidence from five present-day continents — North America, South America, Africa, Australia and Antarctica — in addressing paleogeography at that critical time.

Dalziel proposes that present-day North America was still attached to the southern continents until sometime into the Cambrian period. Current reconstructions of the globe’s geography during the early Cambrian show the ancient continent of Laurentia — the ancestral core of North America — as already having separated from the supercontinent Gondwanaland.

In contrast, Dalziel suggests the development of a deep oceanic gateway between the Pacific and Iapetus (ancestral Atlantic) oceans isolated Laurentia in the early Cambrian, a geographic makeover that immediately preceded the global sea level rise and apparent explosion of life.

“The reason people didn’t make this connection before was because they hadn’t looked at all the rock records on the different present-day continents,” he said.

The rock record in Antarctica, for example, comes from the very remote Ellsworth Mountains.

“People have wondered for a long time what rifted off there, and I think it was probably North America, opening up this deep seaway,” Dalziel said. “It appears ancient North America was initially attached to Antarctica and part of South America, not to Europe and Africa, as has been widely believed.”

Although the new analysis adds to evidence suggesting a massive tectonic shift caused the seas to rise more than half a billion years ago, Dalziel said more research is needed to determine whether this new chain of paleogeographic events can truly explain the sudden rise of multicellular life in the fossil record.

“I’m not claiming this is the ultimate explanation of the Cambrian explosion,” Dalziel said. “But it may help to explain what was happening at that time.”

A new analysis from The University of Texas at Austin’s Institute for Geophysics suggests a deep oceanic gateway, shown in blue, developed between the Pacific and Iapetus oceans immediately before the Cambrian sea level rise and explosion of life in the fossil record, isolating Laurentia from the supercontinent Gondwanaland. – Ian Dalziel

A new analysis of geologic history may help solve the riddle of the “Cambrian explosion,” the rapid diversification of animal life in the fossil record 530 million years ago that has puzzled scientists since the time of Charles Darwin.

A paper by Ian Dalziel of The University of Texas at Austin’s Jackson School of Geosciences, published in the November issue of Geology, a journal of the Geological Society of America, suggests a major tectonic event may have triggered the rise in sea level and other environmental changes that accompanied the apparent burst of life.

The Cambrian explosion is one of the most significant events in Earth’s 4.5-billion-year history. The surge of evolution led to the sudden appearance of almost all modern animal groups. Fossils from the Cambrian explosion document the rapid evolution of life on Earth, but its cause has been a mystery.

The sudden burst of new life is also called “Darwin’s dilemma” because it appears to contradict Charles Darwin’s hypothesis of gradual evolution by natural selection.

“At the boundary between the Precambrian and Cambrian periods, something big happened tectonically that triggered the spreading of shallow ocean water across the continents, which is clearly tied in time and space to the sudden explosion of multicellular, hard-shelled life on the planet,” said Dalziel, a research professor at the Institute for Geophysics and a professor in the Department of Geological Sciences.

Beyond the sea level rise itself, the ancient geologic and geographic changes probably led to a buildup of oxygen in the atmosphere and a change in ocean chemistry, allowing more complex life-forms to evolve, he said.

The paper is the first to integrate geological evidence from five present-day continents — North America, South America, Africa, Australia and Antarctica — in addressing paleogeography at that critical time.

Dalziel proposes that present-day North America was still attached to the southern continents until sometime into the Cambrian period. Current reconstructions of the globe’s geography during the early Cambrian show the ancient continent of Laurentia — the ancestral core of North America — as already having separated from the supercontinent Gondwanaland.

In contrast, Dalziel suggests the development of a deep oceanic gateway between the Pacific and Iapetus (ancestral Atlantic) oceans isolated Laurentia in the early Cambrian, a geographic makeover that immediately preceded the global sea level rise and apparent explosion of life.

“The reason people didn’t make this connection before was because they hadn’t looked at all the rock records on the different present-day continents,” he said.

The rock record in Antarctica, for example, comes from the very remote Ellsworth Mountains.

“People have wondered for a long time what rifted off there, and I think it was probably North America, opening up this deep seaway,” Dalziel said. “It appears ancient North America was initially attached to Antarctica and part of South America, not to Europe and Africa, as has been widely believed.”

Although the new analysis adds to evidence suggesting a massive tectonic shift caused the seas to rise more than half a billion years ago, Dalziel said more research is needed to determine whether this new chain of paleogeographic events can truly explain the sudden rise of multicellular life in the fossil record.

“I’m not claiming this is the ultimate explanation of the Cambrian explosion,” Dalziel said. “But it may help to explain what was happening at that time.”